We propose to use an innovative and noninvasive technique to measure choroidal blood flow in experimental animals and to measure blood flow in experimental intraocular tumors. The blood flow measurement involves the use of a focused external beam of protons to activate the formation in situ of the positron emitting tracer, O15 in the choroidale circulation or in the experimental intraocular tumors. Detection of the gamma rays produced by the subsequent decay of the positron activity induced will be performed. The rate of wash out of O15 is measured and a biologic model which has been developed is used to calculate blood flow in the tissue studied. Because protons have the ability to produce a volume of tracer with relatively sharp boundaries, selective activation of the choroidal circulation and intraocular blood flow is possible. Because this technique is completely noninvasive, repeated and sequential measurements of blood flow may be performed following experimental alterations in ocular perfusion. Using these techniques, we propose to measure choroidal blood flow in the rabbit; blood flow following occlusion of the carotid artery and in the presence of increased intraocular pressure will be measured. An established intraocular tumor model employing the heterotransplantation of Greene melanoma cells into the rabbit choroid will be used in these studies. Experiments to measure tumor blood flow during the development of experimental intraocular melanomas in the rabbit will be performed; growth of the tumors will be documented by wide angle fundus photography and fluorescein angiography. Histopathologic studies of selected tumors will be performed and correlated with blood flow measurements, fundus photography and angiographic appearance.